Multiple compartment packed bed absorber-desorber heat exchanger and method

ABSTRACT

The invention is a multiple compartment packed bed absorberdesorber heat exchanger. The absorber-desorber heat exchanger includes at least one set of upper and lower compartments containing suitable tower packing. Gas inlets, gas outlets and liquid distributors are provided for each packed bed chamber. The gas flows in the upper and lower compartments are isolated from each other by a liquid seal. The upper and lower packed beds are wetted by a common irrigation liquid or heat exchange fluid which gravitates downwardly through the packed beds to a sump and recycling pump. The dual packed beds can be operated to absorb and desorb gases, to concentrate gaseous contaminants, or to transfer heat and to condense vapors. Preferably, the dual packed bed chambers are of modular construction.

[4 1 Feb. 12, 1974 The absorber- 1,278,153 10/1961 France..............

Primary Examiner-Bernard Nozick Attorney, Agent, or Firm-Owen & Owen;Richard D. Emch, Esq.

[57] ABSTRACT The invention is a multiple compartment packed bedabsorber-desorber heat exchanger.

desorber heat exchanger includes at least one set of upper and lowercompartments containing suitable tower packing. Gas inlets, gas outletsand liquid distributors are provided for each packed bed chamber. Thegas flows in the upper and lower compartments are isolated from eachother by a liquid seal. The upper and lower packed beds are wetted by acommon irrigation liquid or heat exchange fluid which gravitatesdownwardly through the packed beds to a sump Richard L. Huntington, Rt.No. 1, Van Buren, Ohio 45889 June 10, 1971 References Cited limitedStates Patent Huntington 1 MULTIPLE COMPARTMENT PACKED BEDABSORBER-DESORBER HEAT EXCHANGER AND METHOD [76] Inventor:

[22] Filed:

[2]] App]. No.: 151,727

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o 0 o o o o o o o o o 0 you 0 IEZ- I.\?\-"ENTOR: FIL'HAED L. HUNTJNETEN.

MULTIPLE COMPARTMENT PACKED BED ABSORBER-DESORBER' HEAT EXCHANGER ANDMETHOD BACKGROUND OF THE INVENTION Packed bed fume scrubbers, commonlyin use, employ a scrubbing liquid to remove gaseous and particulate airpollutants from a gas stream. When the gas stream is hot, cooling of thegas occurs in the scrubber by the evaporation of water from thescrubbing solution with a consequent reduction in absorption efficiencyand a loss bf stack draft are to reduced tern perature and buoyancy.While water vapor by itself is not objectionable chemically, excesswater vapor often concentrates air pollutants in scrubber tail gases.For mp ul u ide. s. th water va srti scrubber tail gases to form asulfuric acid mist which results in critical pollutant concentrationsadjacent the base of an exhaust stack. Under conditions where thescrubbing liquid is recirculated, particulate matter which has beentrapped by the liquid is released when excessive evaporation occurs.

The control of a building air pollution level by ventilation isoftenlirnitedbv the ambient air pollution level. This often contributesto the overall air pollution level, and often results in an increase inheating and cooling plant costs as well as an increase in energy usageand cost. Increased energy usage in turn contributes to the overall airpollution level especially of the energy is supplied by coal or by thecombustion of natural gas.

The recovery of heat from waste gases is often limited by the corrosiveand fouling nature of the air pollutant. Roughness and dirt buildup onthe surface of air heat exchangers results in a large reduction in heattransfer capacity due to a decrease in thermal conduc tance.

SUMMARY OF THE INVENTION This invention relates to packedbed wet fumescrubbers for air pollution control and more particularly to an improvedmultiple chamber cross flow packed bed abso rber-desor ber for therecovery of heat from ex haust gases, the concentration of gaseouscontaminants and the reduction of vapor plume formation when hot gasesare cleaned with water.

A multiple bed cross flow absorber-desorber heat exchanger, according tothe present invention, includes a shell defining an upper and lowerhollow chamber. The chambers are provided with gas inlet-outlets,suitable tower packing and liquid distributors. Gas flow in the upperand lower compartments is isolated from the atmosphere and each other byliquid seals. The packed beds are wetted by a common irrigation liquidwhich gravitates downward through the packed beds to a common sump andrecycling pump. The packed beds may be operated to absorb and desorbboth gaseous contaminants and heat. Heat transfer is accomplished byrecirculating a low vapor pressure heat transfer fluid. Mass transfer isaccomplished by the use of aqueous absorber solutions. The packed bedsare of modular construction and may be stacked and/or mounted in seriesand may be operated to both scrub and recover heat from waste gases.

it is an important object of the present invention to provide a heatexchanger of improved design capable of operating at a high efficiencylevel, while simulta neously presenting minimum impedance to gas flow.

which can be used in a large number of applications and can be producedin large quantities at low cost.

An object of the present invention is to provide an economical processfor the recovery of heat from exhaust gases and the reduction of overallthermal and chemical air pollution level.

Another important object of the present invention is to provide aprocess for the utilization of the adiabatic cooling effect in the crossflow wet fume scrubber to cool and condition make-up ventilation air.

Still a further object of the present invention is to provide aneconomical heat transfer process for increasing absorber tail stack gasbuoyancy. Still an additional object of the present invention is toprovide an economical process for the reduction of vapor plume formationfrom wet scrubbers; the reduction of water usage; and a resultantreduction in air pollution.

Still another important object of the present invention is to provide ameans for concentrating gaseous contaminants by absorption anddesorption utilizing temperature differences and selective solvents.

Still a further object of the present invention is to provide acorrosion resistant and self-cleaning packed Berries? exchanger.

Still additional objects, benefits, and advantages of the presentinvention will become evident from a study of the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a dualchambered cross flow type packed absorber-desorber of modular designshowing the internal construction thereof, made in accordance with thepresent invention;

FIG. 2 is a top view of the absorber shown in FIG. 1;

FIG. 3 is a sectional view of another embodiment of the presentinvention showing the internal construction thereof; 4

FIG. 4 is a top view with parts broken away of the ab sorber-desorbershown in FIG. 3;

FIG. 5 is a sectional view of a modular unit of a packed bedabsorber-heat exchanger used for heat recovery and gas purification;

FIG. 6 is a sectional view of a modular unit of a packed bedabsorber-heat exchanger used for the condensation of water vapor;

FIG. 7 is a sectional view of a modular unit of a cross flow packed bedheat exchanger and absorber for vapor plume suppression and improvedabsorber performance made in accordance with the present invention;

FIG. 8 is a sectional view of a dual packed bed cross flow absorber-heatexchanger with partitions for heat recovery and air purification;

FIG. 9 is a top view of the heat exchanger-absorber shown in FIG. 8,made in accordance with the present invention;

FIG. 10 is a sectional view of a multiple chambered packed bed crossflow heat exchanger absorber according to the present invention;

FIG. 11 is a top view of the heat exchanger absorber cross flow scrubbershown in FIG. 10;

FIG. 12 is a sectional view of a counter flow dual chambered packed bedabsorber-heat exchanger made in accordance with the present invention;and

FIG. 13 is a top view of the counter flow packed bed absorber-heatexchanger shown in FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, adual compartment horizontal cross flow packed bed absorber-heatexchanger according to the present invention is generally indicated bythe reference number 10. The heat exchanger 10 includes upper and lowerpacked bed chambers 11 and 12; gas inlet grills 13 and 14; gas outletgrills l5 and 16; corrugated sieve plate distributors 17 and 18;corrugated side walls 19 and 20; a corrugated packed bed support plate21; a liquid sump pan 22; an auxiliary pump 23; and piping 24 and 25.The corrugated sieveplate liquid distributor 18 serves as a sieve platedistributor for the lower packed bed 12; as a support plate for thepacked bed 11; and as a liquid seal-between the packed beds.

In operation, liquid enters the sieve plate distributor 17 through aliquid inlet port 26 and is distributed across the distributor pan 17forming a liquid seal indicated by the reference number 27. The liquidpasses downward through a plurality of holes 28 in the distributor pan17 (see FIG. 2) into the packed bed chamber 11 where the liquid streamfrom the individual perforations are disbursed by a shallow layer ofsmall diameter packing 29 supported by large diameter packing 30 in thepacked bed 11. The layer of small diameter packing 29 serves both todistribute liquid over the large diameter packing 30 and to increase gasflow resistance along the underside of the distributor pan 17 holdingthe packed bed 11 in compression.

The irrigation liquid flows downward through the packed bed 11 to thedistributor-support plate 18 and collects on the pan to form a liquidseal indicated by the reference number 31. The liquid is redistributedand flows through perforations 32 in the distributor pan 18, through alayer of small diameter packing 33 and packed bed 12, exiting through aplurality of holes 34 in the support plate 21, into the sump pan 22where the liquid collects to form a liquid seal indicated by thereference number 35 along the bottom of the bed 12. The liquid leavesthe sump pan 22 through an outlet port 36 and is returned to thedistributor 17 by the pump 23 and piping 24 and 25.

Gas enters the inlet grills 13 and 14 and passes horizontally throughthe packed beds 11 and 12 coming in contact with down flowing irrigationliquid where heat and "mass trari fer'aeears rhe scrubbed gases exitfromthe packed beds 11 and 12 through the outlet grills l5 and 16.

If the two gas streams differ in temperature and the irrigation liquidconsists of a low vapor pressure heat tiali'rr'fldid such as oil, heatwill be transferred from i the hot gas stream to the oil and from thehot oil to the cold gas stream. The dual packed bed then becomes aneffective heat exchanger and can be used to recover heat from exhaustair, flue gas, and waste gases at moderate temperatures.

If the gas streams differ in humidity and temperature and the irrigationliquid consists of an aqueous dessicant solution, both heat and moisturewill be transferred from one gas stream to the other. The dual packedbed can then be used to suppress the evaporation of liquid therebyreducing heat loss from open proces's armada from wet scrubbers. T

In a method using the FIG. 1 and 2 embodiment, heat and moisture can berecovered from a warm ventilation air stream which enters the upperpacked bed chamber 11. A cold ventilation air stream is introduced inthe lower packed bed chamber 12. Both air streams are passed indifferential contact with a common recycle heat transfer liquid streamcomprising an admixture of liquid paraffin and polypropylene glycol. Theliquid stream is distributed to the chambers 11 and 12 by thedistribution pan 17. The liquid stream continuously absorbs heat andmoisture from the warm ventilation air by contact with the heat transferliquid in the chamber 11 and continuously desorbs the heat and moistureto the cold ventilation air stream by contact with the heat transferliquid stream in the lower chamber 12.

FIGS. 3 and 4 show another embodiment of a dual bed packed bedabsorber-desorber heat exchanger 40. The heat exchanger 40 includes anupper packed bed chamber 41 and a lower packed bed chamber 42 defined byside walls 43 and 44; an upper sieve plate distributor pan 45; gasinlet-outlet plenum chambers 46, 47, 48 and 49; an intermediate sieveplate partition 50; a bottom wall 51; a recirculation pump 52; andinterconnecting pipes 53 and 54. In operation, liquid enters adistributor pan inlet port 55 and is distributed over the distributorpan 45 forming a liquid seal. The liquid flows downwardly into the upperpacked bed chamber 41, through a plurality of holes 56 defined by thedistributor pan 45. The liquid passes over the packing, which consistsof criss-cross corrugated sheeting, and collects on the sieve platepartition 50 forming a liquid seal 57. The liquid flows through holes 58in the perforated partition 50 and downward over the surface of the bed42 to a base 59 of the exchanger 40, where it is returned to the topdistributor 45 by the pump 52 and piping 53 and 54. Gas streams enterinlets 60 and 61. The gas flows through the packed beds 41 and 42, wheremass-and heat transfer occurs by direct contact with the irrigationliquids, and exits through the outlets 62 and 63.

Wall corrugations and inlet-outlet grills are unnecessary in the designshown in FIG. 3 because the packing consists of evenly spacedself-supporting modular crisscross sheets which redistribute theirrigation liquid. The sheets are parallel to both the liquid and thegas flow. Entwined wire mesh packing can be used with efficiency as apacking. The tower fill preferably should be made from materials whichare thermally conductive, corrosion resistance and which are easilywetted by the irrigation liquid. The heat transfer fluid should have ahigh thermal conductance; exhibit a low vapor pressure over theoperating temperature range; be fire resistant and be low in viscosity.For high temperature ranges, from 400 to 500F., the heat transfer fluidcan consist of a plastic polymer dissolved in a low vapor pressureplasticizer.

A modular unit 70 is shown in FIG. 5. The unit 70 includes four crossflow packed bed chambers 71, 72, 73 and 74; auxiliary pumps 75 and 76;and piping 77-84. The unit 70 is designed for the purpose of recoveringheat and purifying ventilation air. When heat is to be receovered fromexhaust gases in winertime, the irrigation liquid in the lead packed bedcompartments 71 and 72 consists of heat transfer fluid which isrecirculated through the packed bed chambers 71 and 72 through the pump75 and interconnecting piping 77, 78 and 80. The back packed bedcompartments 73 and 74 are irrigated with an aqueous'dessicant solutionwhen the latent heat of vaporization is to be recovered from the exhaustgas or a chemical solution when the air is to be purified. The aqueoussolution is circulated through the packed bed chambers 73 and 74 by thepump 76 and piping 81, 82 and 84 or through the bypass piping 83' whenthe incoming I air does not require purification.

When refrigeration is to be recovered from exhaust ventilization air insummertime, the lead packed bed chambers 71 and 72 are preferentiallyoperated with aqueous solutions and the back packed bed chambers 73 and74 are operated with heat transfer fluid so as to be able to takeadvantage of adiabatic cooling caused by the evaporation of water fromthe scrubbing solution which results in a net gain in refrigeration ifthe lead packed bed chamber 71 is operated below capacity. The exhaustgas to be cleaned is preferentially passed through the lower packed bedchambers 72 and 74.

Another modular unit 90 is shown in FIG. 6. The unit 90 includes twocross flow packed bed chambers 91 and 92; an auxiliary pump 93; piping94 and 95; and a flow condensate overflow 96. The unit 90 is used forcondensing water vapor and for the elimination of vapor plumes from wetscrubbers. Heat transfer fluid is circulated in the packed beds 91 and92 through the pump 93 and piping 94 and 95. Cold air enters the packedbed 91, cooling the down flowing heat transfer fluid. The cold air isheated by the fluid and combines with cold dehumidified air which entersthe lower packed bed chamber 92 as warm saturated air. The water vaporis condensed from the warm saturated air by direct contact with downflowing cold heat transfer fluid. The condensate and heat transfer fluidare decanted in a base sump 97. The excess condensate is removed fromthe sump 97 through the overflow 96 and the heat transfer fluid isrecycled to the packed bed cooling chamber 91 through the pump 93 andthe piping 94 and 95.

FIG. 7 shows a unit 100 having three modular cross flow packed bedchambers for the purpose of precooling absorber gas, absorbing andreheating absorber tail gas. The unit 100 includes a packed bed gasprecooling chamber 101, a packed bed absorption chamber 102, a packedbed absorber tail gas heating chamber 103, auxiliary pumps 104, 105 and106, and piping 107l13. In operation, the warm contaminated gas passeshorizontally through the packed bed chamber 101 and is cooled by downflowing cold heat transfer fluid, which is warmed by the gas andcollects in a sump pan 114 where it is returned to the packed bedabsorber tail gas reheat chamber 103 by the pump 104 through theinterconnecting piping and 111. The cooled gas passes through the packedbed absorption chamber 102 into the tail gas' reheat chamber 103 whereit is reheated by the warm heat transfer fluid flowing downward throughthe packed bed to a sump chamber 115. The cooled heat transfer fluid isreturned to the packed bed gas cooling chamber 101 by the pump 105 andinterconnecting piping 113 and 109. The pump 106 and interconnectingpiping 107 and 108 serve to recycleabsorber solution while the piping112 compensates for unequal recycle pump capacity by equalizing theliquid levels in the heat transfer fluid sumps 114 and 115.

The heat extracted from hot gases may also be used in the desorption ofgases by direct absorption solution and heat transfer fluid contact orused as a source of thermal energy.

FIGS. 8 and 9 disclose a dual bed absorber employing chevron-shapedpartitions 121 and 122 to partition the dual packed beds 123 and 124into compartments 125, 126, 127 and 128. Partitioned distributors 129and 130 and partitioned sumps 131 and 132 are also provided. Theabsorber 120 alsoincludes auxiliary pumps 133 and 134, and piping135-142. The partitioned dual bed packed bed absorber 120 is analternate embodiment to the unit 70 (FIG. 5).

A multiple chambered cross flow packed bed absorber is shown in FIGS. 10and 11. The absorber 150 includes a packed bed 151 which is divided bychevron partitions 152153 into a packed bed gas cooling chamber 154, apacked bed absorption chamber 155, and an absorber tail gas reheatpacked bed chamber 156. The absorber 150 also includes a partitioneddistributor pan 157; partitioned sumps 158, 159 and 160; auxiliary pumps161, 162 and 163; and piping 164-170. The absorber 150 is an alternateembodiment to the modular absorber unit 100 shown in FIG. 7.

FIGS. 12 and 13 show another embodiment of the present invention. A unitemploys counter flow packed bed chambers for heat transfer. The unit 180includes a sump and gas inlet chamber 181, a slanting packed bed supportgrid 182, a packed bed chamber 183, a slanting packed bed hold down grid184, a gas outlet chamber 185, a mist eliminator filter outlet 186, aliquid sieve plate distributor partition 187,-a gas inlet plenum andsump chamber 188, a slanting packed bed support plate grill 189, aslanting packed bed chamber 190, a slanting packed bed hold down grill191, a gas outlet plenum chamber 192, a mist eliminator filter outlet193, a sieve plate distributor pan 194, an auxiliary pump 195, andpiping 196 and 197.

In operation, gas enters the inlet chamber 181, passes upward throughthe slanting support plate grid 182, packed bed chamber 183, packed bedhold down grill 184 and into the gas outlet chamber 185. The gas thenpasses out of the outlet chamber 185 through the mist eliminator filteroutlet 186. Heat transfer fluid enters the gas outlet chamber 185through perforations 198 in the sieve plate partition 187 and passesdownward through the packed bed hold down grid 184; packed bed chamber183; support grid 182; inlet plenum chamber 181; and collects in a sump199. The fluid is returned to the upper distributor pan 194 by the pump195 and interconnecting piping 196 and 197. Gas entering the upper gasinlet plenum chamber 188 passes upward through the slanting packed bedsupport plate 189, through the packed bed 190, through the hold downgrid 191 and into the gas outlet chamber 192. The gas exits through thegas outlet mist eliminator outlet 193. Heat transfer fluid entering thesieve plate distributor pan 194 passes through a plurality of holes 200in the pan 194; through the gas outlet plenum chamber 192; through theslanting hold down grid 191; through the packed bed chamber 190; throughthe slanting support plate 'grill 189; through the gas inlet chamber 188and collects on the sieve plate distributor partition 187. This forms aliquid seal 201. The fluid flows through the perforations 198 into thegas outlet chamber 185. The direction of heat and mass transfer isdependent upon the temperature difference and concentration gradient.

While the above packed bed heat exchanger and absorber designs,according to the present invention, have been described with particularreference to the embodiments shown in the drawings, many changes may bemade in the detailed construction. These include, for examples, the useof distributor covers, slanting the distributor and packed bed tocompensate for the pressure drop across the cross flow packed bed, andthe use of permanent liquid weir box trough distributor seals. v

The operation of an absorber at reduced temperatures by means ofexternal refrigeration, and by the recovery of refrigeration from theabsorber tail gas, can drastically reduce absorber packed bed volume.

Additional advantages will become apparent from a review of the appendedclaims.

What 1 claim is:

l. A multiple compartment absorber-desorber heat exchanger comprising,in combination, shell means including an upper rectangular housingcircumjacent a lower rectangular housing for defining at least one upperpacked bed chamber and at least one lower packed bed chamber, said upperand lower packed bed chambers having packed material therein, firstdistributor means including a rectangular distributor plate adjacent theupper portion of said upper packed bed chamber for distributing a heatexchange liquid downwardly through said upper chamber, seconddistributor means including a rectangular distributor plate spacedbetween said upper packed bed chamber and said lower packed bedchamberfor distributing the heat exchange liquid downwardly through saidlower packed bed chamber, first opposed inlet and outlet means definedin opposed sides of said shell means adjacent said upper packed bedchamber for directing gas in a path transverse to the path of the heatexchange liquid, second opposed inlet and outlet means defined inopposed sides of said shell means adjacent said lower packed bed chamberfor directing gas in a path opposed to the gas path through the upperpacked bed chamber and transverse to the path of the heat exchangerliquid, a sump positioned below said lower packed bed chamber and meansfor recirculating heat exchange liquid from said sump to said firstdistributor means.

2. A multiple compartment horizontal cross flow packed bedabsorber-desorber heat exchanger comprising, in combination, upper andlower hollow shells, said shells being upwardly open and verticallyaligned, said shells having side walls, corrugated bottom walls, andopposed inlet and outlet means, and partition means for defining upperand lower packed bed compartments, said packed bed compartmentsincluding packing material means for distributing liquid downwardlythrough said upper and lower packed bed compartments and means forcollecting the liquid.

3. A multiple compartment horizontal cross flow absorber-desorber heatexchanger as set forth in claim 2, wherein said packing material in saidpacked bed chamber includes an upper layer of small diameter towerpacking and a lower layer of large diameter tower packing. 7

4. A multiple compartment horizontal cross flow absorber-desorber heatexchanger as set forth in claim 2, wherein said partition meanscomprises grills having spaced cross members, said grills being tiltedin the direction of gas flow.

5. A multiple compartment horizontal cross flow packed bedabsorber-desorber heat exchanger as set forth in claim 2, wherein saiddistributing means includes a flow distributor having a corrugatedbottom wall, with said distributor being positioned on said upper shellabove said upper packed bed compartment.

6. A multiple compartment horizontal cross flow packed bedabsorber-desorber heat exchanger as set forth in claim 5, wherein saiddistributing means includes a perforated distributor for distributingthe liquid to said lower shell, said perforated distributor beingpositioned between said upper and lower hollow shells.

7. A method of recovering heat and moisture from warm ventilation aircomprising the steps of irrigating with a common recycled heat transferliquid stream two or more stacked packed bed chambers, whereby a warmventilation air stream and a cold ventilation air stream are passed indifferential contact with the com mon recycled heat transfer liquidstream comprising an admixture of liquid paraffin and polypropyleneglycol, continuously absorbing heat and moisture from said warmventilation air by contact with the heat transfer liquid stream in anupper packed bed chamber and continuously desorbing said heat andmoisture to the cold ventilation air stream by contact with the heattransfer liquid stream in a lower packed bed chamber.

2. A multiple compartment horizontal cross flow packed bedabsorber-desorber heat exchanger comprising, in combination, upper andlower hollow shells, said shells being upwardly open and verticallyaligned, said shells having side walls, corrugated bottom walls, andopposed inlet and outlet means, and partition means for defining upperand lower packed bed compartments, said packed bed compartmentsincluding packing material means for distributing liquid downwardlythrough said upper and lower packed bed compartments and means forcollecting the liquid.
 3. A multiple compartment horizontal cross flowabsorber-desorber heat exchanger as set forth in claim 2, wherein saidpacking material in said packed bed chamber includes an upper layer ofsmall diameter tower packing and a lower layer of large diameter towerpacking.
 4. A multiple compartment horizontal cross flowabsorber-desorber heat exchanger as set forth in claim 2, wherein saidpartition means comprises grills having spaced cross members, saidgrills being tilted in the direction of gas flow.
 5. A multiplecompartment horizontal cross flow packed bed absorber-desorber heatexchanger as set forth in claim 2, wherein said distributing meansincludes a flow distributor having a corrugated bottom wall, with saiddistributor being positioned on said upper shell above said upper packedbed compartment.
 6. A multiple compartment horizontal cross flow packedbed absorber-desorber heat exchanger as set forth in claim 5, whereinsaid distributing means includes a perforated distributor fordistributing the liquid to said lower shell, said perforated distributorbeing positioned between said upper and lower hollow shells.
 7. A methodof recovering heat and moisture from warm ventilation air comprising thesteps of irrigating with a common recycled heat transfer liquid streamtwo or more stacked packed bed chambers, whereby a warm ventilation airstream and a cold ventilation air stream are passed in differentialcontact with the common recycled heat transfer liquid stream comprisingan admixture of liquid paraffin and polypropylene glycol, continuouslyabsorbing heat and moisture from said warm ventilation air by contactwith the heat transfer liquid stream in an upper packed bed chamber andcontinuously desorbing said heat and moisture to the cold ventilationair stream by contact with the heat transfer liquid stream in a lowerpacked bed chamber.